Which animals were especially heroic in Animal Farm?

Boxer, the cart horse, is the most heroic of the animals. He is a true believer in the ideals of Animal Farm and works incredibly hard for the betterment of the whole community. A very strong horse, he gets up a half hour before everyone else to begin work. He also volunteers for the hardest tasks. His motto when he faces any obstacle is always to "work harder." 
He is a brave fighter. Yet, when he accidentally kills a stable boy in the Battle of the Cowshed, he feels remorse. A large part of what makes him heroic is his moral decency. He truly cares about others, and unlike the pigs, he is not simply trying to help himself. He wants to work for the common good. Sadly, the pigs exploit him mercilessly.
I would also call the hens especially heroic when they stand up for their eggs. Napoleon has entered a contract that requires selling their eggs. They protest by perching on a high rafter in the barn. They lay their eggs there so that they roll off and smash on the ground. Napoleon goes after them ruthlessly—he cuts off their food supply—but nine of them die rather than capitulate. Napoleon does win, but not until after the hens have made a brave stand. 

In Animal Farm, Orwell portrays several animals as being heroic. Towards the beginning of the novella, Snowball demonstrates his courage during the Battle of the Cowshed by fearlessly leading the animal charge against Jones and his men. During the battle, Snowball is shot and wounded by Mr. Jones yet continues to fight until the humans are banished from the farm. During their ceremony after the battle, Snowball and Boxer are awarded the military decoration "Animal Hero, First Class." Unfortunately, Snowball is later driven from the farm by Napoleon and labeled a traitor.
Another animal that displays his heroics in the novella is Boxer, the unintelligent, hard-working cart horse. Boxer not only helps the animals during the Battle of the Cowshed but also does the majority of the difficult work on the farm. Orwell portrays Boxer's work ethic by illustrating how he single-handedly drags thousands of pounds of stone from the quarry to build the windmill. Unfortunately, Boxer does not get to enjoy retirement and essentially works himself to death. Napoleon ends up sending Boxer to the knackers in one of the most heartbreaking scenes in the story.